David Anderson 2685260ba1 net/dns: add temporary fallback to quad-9 resolver for split-DNS testing.
This allows split-DNS configurations to not break clients on OSes that
haven't yet been ported to understand split DNS, by falling back to quad-9
as a global resolver when handed an "impossible to implement"
split-DNS config.

Part of #953. Needs to be removed before shipping 1.8.

Signed-off-by: David Anderson <danderson@tailscale.com>
2021-04-08 23:26:22 -07:00
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